This invention relates to a multiplex transmission system and, more particularly, to a multiplex transmission system for connecting a plurality of control units in a vehicle to a multiplex transmission line and performing an exchange of information among the control units.
In a multiplex transmission system which employs decentralized multiplexing for transmitting and receiving signals between electrical accessories mounted in a vehicle, plural items of data are multiplexed on a time-shared basis on a pair of transmission lines, and serial transmission is the fundamental type of transmission used.
In a multiplex transmission system employing such decentralized multiplex transmission, each node has its own transmission LSI, microprocessor or the like, and these perform such operations as transmission control and analysis of received information in accordance with prescribed algorithms.
However, since each node transmits data, which is necessary for control, at fixed intervals in the prior art described above, a problem which arises is that it is difficult for another node which performs another kind of control to interrupt the data transmission, and therefore a delay develops in this control. In addition, in a case where the period of a data transmission is lengthened, the period for updating the data also is lengthened as a result, and this makes it easy for transmission errors and control delay to occur.
In a case where a data transmission is performed periodically between nodes upon deciding order of priority, a problem which arises is that, depending upon the travelling state of the vehicle, the frequency of data transmission with nodes having a low order of priority rises, as a result of which a data transmission with these nodes either cannot be carried out or is delayed.
Furthermore, since each node transmits data necessary for control at fixed intervals (i.e., at a fixed period), as mentioned above, the density of data on a transmission line is constant at all times regardless of whether the data is important or not in terms of control. In particular, if the amount of data having comparatively little importance transmitted on a transmission line occupies a major share of the line, the probability of performing a transmission of data having a high degree of importance declines, and this causes a delay in control relating to this data.
Another problem is that after the ignition of a vehicle is turned on (IG-ON), the voltage supplied to each control unit declines owing to consumption of power by the starter during starting of the engine, and the transmission operation is destabilized as a result. Though this destabilizing influence upon the transmission operation becomes particularly pronounced at cold starting of the engine, the following problems also arise if the unit which administers engine control receives this destabilizing influence even when the engine is not being cold-started:
(1) Control at starting cannot be performed normally. This may make it difficult or impossible to start the engine or may cause a delay in starting.
(2) Checking of transmitted and received data (control of an ACK signal, which is a reception acknowledgement signal) at the control unit cannot be performed normally.
In a case where a specific control unit undergoes system reset owing to an abnormality in the power-supply system during travelling of the vehicle, the state of the engine or control unit will be unstable even if the power-supply system is restored to normal. Consequently, reception of a signal from another control unit cannot be performed reliably, and engine control cannot be restored or develops a delay.